Photoacoustic breast scanner

ABSTRACT

Methods and apparatus for measuring and characterizing the localized electromagnetic wave absorption properties of biologic tissues in vivo, using incident electromagnetic waves to produce resultant acoustic waves. Multiple acoustic transducers are acoustically coupled to the surface of the tissue for measuring acoustic waves produced in the tissue when the tissue is exposed to a pulse of electromagnetic radiation. The multiple transducer signals are then combined to produce an image of the absorptivity of the tissue, which image may be used for medical diagnostic purposes. In specific embodiments, the transducers are moved to collect data from multiple locations, to facilitate imaging. Specific arrangements of transducers are illustrated. Also, specific mathematical reconstruction procedures are described for producing images from transducer signals.

FIELD OF THE INVENTION

The present invention relates to imaging properties of tissue based upondifferential absorption of electromagnetic waves in differing tissuetypes by photo-acoustic techniques.

BACKGROUND OF THE INVENTION

It is well established that different biologic tissues displaysignificantly different interactions with electromagnetic radiation fromthe visible and infrared into the microwave region of theelectromagnetic spectrum. While researchers have successfully quantifiedthese interactions in vitro, they have met with only limited successwhen attempting to localize sites of optical interactions in vivo.Consequently, in vivo imaging of disease at these energies has notdeveloped into a clinically significant diagnostic tool.

In the visible and near-infrared regions of the electromagneticspectrum, ubiquitous scattering of light presents the greatest obstacleto imaging. In these regions, scattering coefficients of 10-100 mm⁻¹ areencountered. Consequently, useful numbers of unscattered photons do notpass through more than a few millimeters of tissue, and imagereconstruction must rely on multiply-scattered photons. While effortspersist to use visible and infrared radiation for imaging through thicktissue (thicker than a few centimeters), clinically viable imaginginstrumentation has not been forthcoming.

In the microwave region (100-3000 MHz), the situation is different.Scattering is not as important, since the wavelength (in biologictissue) at these frequencies is much greater than the "typical"dimension of tissue inhomogeneities (≈1 μm). However, the offsettingeffects of diffraction and absorption have forced the use of longwavelengths, limiting the spatial resolution that can be achieved inbiologic systems. At the low end of the microwave frequency range,tissue penetration is good, but the wavelengths are large. At the highend of this range, where wavelengths are shorter, tissue penetration ispoor. To achieve sufficient energy transmission, microwave wavelengthsof roughly 2-12 cm (in tissue) have been used. However, at such a longwavelength, the spatial resolution that can be achieved is no betterthan roughly 1/2 the microwave length, or about 1-6 cm.

In vivo imaging has also been performed using ultrasound techniques. Inthis technique, an acoustic rather than electromagnetic wave propagatesthrough the tissue, reflecting from tissue boundary regions where thereare changes in acoustic impedance. Typically, a piezoelectric ceramicchip is electrically pulsed, causing the chip to mechanically oscillateat a frequency of a few megahertz. The vibrating chip is placed incontact with tissue, generating a narrow beam of acoustic waves in thetissue. Reflections of this wave cause the chip to vibrate, whichvibrations are converted to detectable electrical energy, which isrecorded.

The duration in time between the original pulse and its reflection isroughly proportional to the distance from the piezoelectric chip to thetissue discontinuity. Furthermore, since the ultrasonic energy isemitted in a narrow beam, the recorded echoes identify features onlyalong a narrow strip in the tissue. Thus, by varying the direction ofthe ultrasonic pulse propagation, multi-dimensional images can beassembled a line at a time, each line representing the variation ofacoustic properties of tissue along the direction of propagation of oneultrasonic pulse.

For most diagnostic applications, ultrasonic techniques can localizetissue discontinuities to within about a millimeter. Thus, ultrasoundtechniques are capable of higher spatial resolution than microwaveimaging.

The photoacoustic effect was first described in 1881 by Alexander GrahamBell and others, who studied the acoustic signals that were producedwhenever a gas in an enclosed cell is illuminated with a periodicallymodulated light source. When the light source is modulated at an audiofrequency, the periodic heating and cooling of the gas sample producedan acoustic signal in the audible range that could be detected with amicrophone. Since that time, the photoacoustic effect has been studiedextensively and used mainly for spectroscopic analysis of gases, liquidand solid samples.

It was first suggested that photoacoustics, also known asthermoacoustics, could be used to interrogate living tissue in 1981, butno subsequent imaging techniques were developed. The state of prior artof imaging of soft tissues using photoacoustic, or thermoacoustic,interactions is best summarized in Bowen U.S. Pat. No. 4,385,634. Inthis document, Bowen teaches that ultrasonic signals can be induced insoft tissue whenever pulsed radiation is absorbed within the tissue, andthat these ultrasonic signals can be detected by a transducer placedoutside the body. Bowen derives a relationship (Bowen's equation 21)between the pressure signals p(z,t) induced by the photoacousticinteraction and the first time derivative of a heating functions,S(z,t), that represents the local heating produced by radiationabsorption. Bowen teaches that the distance between a site of radiationabsorption within soft tissue is related to the time delay between thetime when the radiation was absorbed and when the acoustic wave wasdetected.

Bowen discusses producing "images" indicating the composition of astructure, and detecting pressure signals at multiple locations, but thegeometry and distribution of multiple transducers, the means forcoupling these transducers to the soft tissue, and their geometricalrelationship to the source of radiation, are not described.Additionally, nowhere does Bowen teach how the measured pressure signalsfrom these multiple locations are to be processed in order to form a 2-or 3-dimensional image of the internal structures of the soft tissue.The only examples presented are 1-dimensional in nature, and merelyillustrate the simple relationship between delay time and distance fromtransducer to absorption site.

SUMMARY OF THE INVENTION

The present invention improves upon what is disclosed by Bowen in twoways. First, the present invention uses multiple transducers to collectphotoacoustic signals in parallel, and then combines these signals toform an image. This approach represents a significant advance over Bowenin that the use of multiple, parallel transducers, substantially reducesthe time needed to collect sufficient information for imaging.Furthermore, while Bowen fails to suggest methodologies for creatingmultidimensional images, the present invention provides specificmethodologies for reconstructing multidimensional images of internaltissues through the combination of multiple pressure recordings. As partof achieving these advances over Bowen, the present invention detailsthe frequencies that might be used, the size of the multipletransducers, their geometrical relationship to one another and to thetissue, and structures for coupling sensors to the tissue.

Specifically, in one aspect, the invention provides a method of imagingtissue structures by detecting localized absorption of electromagneticwaves in the tissue. An image is formed by irradiating the tissue with apulse of electromagnetic radiation, and detecting and storing resultantpressure waveforms arriving at the acoustic sensors. Multiple detectedpressure waveforms are then combined to derive a measure of the extentto which pressure waveforms are originating at a point distant from theacoustic sensors. This step can then be repeated for multiple points toproduce an image of structures in the tissue.

In a disclosed particular embodiment, the multiple pressure waveformsare combined to form an image at a point by determining a distancebetween the point and a pressure sensor, and then computing a valuerelated to the time rate of change in the pressure waveform, at a timewhich is a time delay after the pulse of electromagnetic radiation--thistime delay being equal to the time needed for sound to travel throughthe tissue from the point to the pressure sensor. This process,determining a distance and time delay, and then computing a value fortime rate of change, is repeated for each additional pressure sensor andits pressure waveform, and the computed values are accumulated to formthe measure of the pressure waveforms originating at the point. Thesepoint measurements may then be collected into a multi-dimensional image.

In one specific embodiment, the pressure sensor signal is processed byappropriate electrical circuitry so that the electrical output of thesensor is representative of the time rate of change of the pressurewaveform. As a result, the value representing the time rate of change ofpressure is directly available from the sensor output. To create anappropriate output, delayed versions of the output of the sensor arecombined with the output of the sensor, which produces an electricaloutput representative of the time rate of pressure change.

In an alternative embodiment discussed below, a measure of pressurewaveforms originating at a point, is generated by computing a valuerelated to a sum of the pressure waveform detected by the acoustictransducer over the time period--where again the time period beginssimultaneous with the electromagnetic irradiating pulse, and has aduration equal to the time needed for sound to travel through the tissuefrom the point to the pressure sensor. These steps can then be repeatedfor additional pressure sensors and their waveforms, and the resultsaccumulated as discussed above to form the measure of pressure waveformsoriginating at the point.

In either approach, it is useful to multiply the computed time rate ofchange, or computed time period sum, of an acoustic transducer signal,by a factor proportional to the time delay used to produce the value.Doing so compensates for the diffusion of acoustic energy radiated fromthe point as it travels through the tissue to the transducer.

In apparatus for carrying out these imaging methods, the sensors arepositioned on a surface and relatively evenly spaced across the surfaceso as to, in combination, produce sharp multi-dimensional images throughthe tissue. To reduce the number of sensors required, the sensors may bemoved to multiple positions while producing an image. Specifically,while the sensors are in a first position, the tissue is irradiated andthe pressure waveforms from the sensors are recorded. Then the sensorsare moved to a second position and the irradiation and waveform storageare repeated. In this way, each sensor can be moved to a number ofpositions to generate multiple waveforms. All of the stored waveformscan then be combined to generate an image of the tissue.

The sensors may be positioned on a plane and moved in a rectilinearfashion, in which case the electromagnetic irradiation source may bemoved in synchrony with the sensors. Alternatively, the sensors may bepositioned on a spherical surface (having a center of curvatureapproximately in the center of the tissue region to be imaged) which isrotated to multiple positions. In this latter case, the sensors can beadvantageously positioned on the spherical surface along a spiral path,so that rotation of the sensors produces a relatively even distributionof sensor locations across the spherical surface.

To enhance acoustic coupling to the tissue, the sensors may be immersedin an acoustic coupling media, having an acoustic characteristicimpedance which is substantially similar to that of the tissue to reducereflections of acoustic waves impinging into the media from the tissue.A flexible film may be used to contain the acoustic coupling media, sothat the tissue can be pressed upon the flexible film to couple acousticwaves from the tissue into the acoustic coupling media.

Similarly, the electromagnetic radiation source may be immersed in anelectromagnetic coupling media having an electromagnetic characteristicimpedance which is substantially similar to that of the tissue to reducereflections of electromagnetic waves impinging into the tissue from theelectromagnetic coupling media. Here again the tissue can be pressedupon the flexible film to couple electromagnetic waves from theelectromagnetic coupling media into the tissue.

In one particular embodiment, both the electromagnetic radiation sourceand the acoustic transducers are immersed in the same coupling media,and the coupling media has a characteristic acoustic and electromagneticimpedance which is substantially similar to that of the tissue.

The electromagnetic radiation may be laser-generated radiation in theultraviolet, visible or near-infrared band, light generated by a Xenonflash lamp, or microwave frequency radiation from a microwave antennasuch as a coil. In the latter case, a microwave frequency of fourhundred and thirty-three or nine hundred and fifteen MHz may beadvantageous since these frequencies are FCC approved and fall within afrequency band in which malignant and normal tissue exhibitsubstantially different absorptivities.

The above and other objects and advantages of the present inventionshall be made apparent from the accompanying drawings and thedescription thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate embodiments of the invention and,together with a general description of the invention given above, andthe detailed description of the embodiments given below, serve toexplain the principles of the invention.

FIG. 1 is a functional block diagram of a photoacoustic scanner forscanning breast tissue in accordance with a first embodiment of thepresent invention;

FIG. 2 is a top view of one embodiment of a transducer array for thescanner of FIG. 1;

FIG. 3 illustrates the waveforms produced in the scanner of FIG. 1;

FIG. 4 illustrates the spatial response of pressure transducers used ina photoacoustic scanner such as that of FIG. 1;

FIG. 5 is a second embodiment of a photoacoustic breast scanner inaccordance with the present invention, using a laser or flash tubesource of electromagnetic energy;

FIG. 6 is an embodiment of a transducer array and electromagnetic sourcefor a scanner such as that of FIG. 1, configured for rectilinearscanning motion;

FIG. 7 is an embodiment of a transducer array and electromagnetic sourcefor a scanner such as that of FIG. 1, configured for rotational scanningmotion;

FIG. 8 is a particular embodiment of a rotationally scanning transducerarray, formed on a spherical surface, illustrating the positioning ofthe transducers on the spherical surface of the array;

FIG. 9 illustrates the axial alignment of the transducers on thespherical surface of the array of FIG. 8;

FIG. 10 illustrates the locus of transducer positions brought aboutthrough rotational scanning of the array of FIG. 8;

FIGS. 11A and 11B are a third embodiment of a photoacoustic breastscanner in accordance with the present invention, using an acousticcoupling tank configured to permit placement of a rotationally scanningacoustic transducer array in close proximity to a human breast;

FIG. 12 is a circuit diagram of an integral transducer signal amplifierfor a photoacoustic breast scanner;

FIG. 13 is a fourth embodiment of a photoacoustic breast scanner inaccordance with the present invention, using an acoustic coupling tankconfigured to permit a rotationally scanning acoustic transducer arrayto surround a human breast;

FIG. 14 illustrates the geometric relationships involved in thereconstruction methodologies used to generate a tissue image;

FIG. 15 illustrates a reconstruction methodology for forming a tissueimage from acoustic transducer signals;

FIG. 16 is an experimental apparatus used to generate an image of anabsorption phantom generally in accordance with the methodology of FIG.15, and FIG. 17 is the image created therefrom;

FIG. 18 illustrates a second reconstruction methodology for forming atissue image from acoustic transducer signals;

FIG. 19 illustrates the ideal impulse response of a transducer whichproduces an electrical output signal indicative of the first temporalderivative of an incident pressure signal;

FIGS. 20A-20D illustrate a simulated actual impulse response and amethodology for converting this impulse response to an approximation ofthe ideal response illustrated in FIG. 19; and

FIG. 21 is a circuit diagram of a circuit for performing the conversionmethodology of FIG. 20.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

FIG. 1 illustrates a photoacoustic breast scanner 10 in accordance withone embodiment of the present invention, which displays several keyelements for successful photoacoustic scanning of the female humanbreast.

A human breast 12 is compressed between two coupling tanks 14, 16.Coupling tank 14 contains fluid or semi-solid media 18 having dielectricproperties which are close to that of "average" breast tissue at themicrowave (or radio wave) frequencies used to stimulate photoacousticemission within the breast 12. Examples would be salinated water,alcohol or mineral oil. The media 18 is contained within tank 14 by aflexible sheet 19, for example of polyethylene, on the surface of thetank coupled to the breast 12. Sheet 19 ensures good mechanical contactbetween the tissue of breast 12 and the media 18 in tank 14.

Within the top coupling tank is a microwave antenna 20. A microwavegenerator 22, i.e., a source of pulse microwave or radio wave energy, iscoupled to antenna 20 through a transmission line 24. (One suitablemicrowave generator is a Hewlett-Packard model 8657B tunable generator,coupled to a 200 Watt RF amplifier available from AMP Research. Antenna20 is large enough to irradiate all or a large fraction of the breastvolume to be imaged. A cylindrically-shaped coil antenna, three to nineinches in diameter would be suitable. Further details on waveguideswhich can be used as microwave radiators can be found in Fang et al.,"Microwave Applicators for Photoacoustic Ultrasonography", Proc. SPIE2708: 645-654, 1996, which is incorporated by reference herein in itsentirety.

The purpose of dielectric coupling media 18 and sheet 19 is to improvethe penetration of the microwave energy into the breast tissue. Becausebreast 12 is compressed against the surface of tank 14, there is acontinuous interface between coupling media 18 and the tissue of breast12, uninterrupted by air gaps. An air gap, or any other physicaldiscontinuity having a corresponding discontinuity in dielectricproperties, will cause a large fraction of the microwave energy toreflect away from the interface (and thus away from the surface of thebreast), rather than penetrate into the breast. By matching thedielectric properties of the breast and media 18, and eliminating airgaps, such discontinuities are reduced, improving microwave penetrationinto breast 12.

As noted above, microwave generator 22 delivers short-duration pulses ofradiation to breast 12. These bursts should last anywhere from 10nanoseconds to one microsecond, e.g., 0.5 microseconds. Each radiationburst causes localized heating and expansion of the breast tissueexposed to the microwave energy. Tissue heating and expansion will begreatest in those regions of the breast tissue which are most absorptiveof the microwave energy. If a region of tissue within breast 12 (e.g., atumor) is particularly more absorptive than the surrounding tissue, theregion will expand relatively more rapidly and extensively than thesurrounding tissue, creating an acoustic wave which will propagatethrough the tissue. These acoustic waves are manifested as longitudinalpressure waves, containing acoustic frequencies ranging from very lowfrequencies to approximately the reciprocal of the electromagnetic pulselength. For a one-half microsecond irradiation pulse, this maximumacoustic frequency would be 2 million cycles per second, or twomegahertz (MHz).

Any of several different microwave frequencies may be used, butfrequencies in the range of 100-1000 MHz are likely to be particularlyeffective. At these frequencies, energy penetration is good, absorptionis adequate, and differential absorption between different types oftissue, e.g. fat and muscle, is high. It has also been reported that theratio of absorbed energy in cancerous relative to normal breast tissueis enhanced in this frequency range, peaking at 2-3 between about300-500 MHz. (See, e.g., Joines, W. T. et al, "The measured electricalproperties of normal and malignant human tissues from 50-900 MHz",Medical Physics, 21(4):547-550, 1994.) The frequency of 433 MHz,specifically, has been approved by the FCC for use in hyperthermiatreatments, and accordingly is available and may be used inphotoacoustic imaging in accordance with the present invention. Imagingmight also be performed at the FCC approved frequency of 915 MHz.Furthermore, it has been reported that the electrical conductivity ofmalignant tissue and normal tissue may vary by a factor of fifty.Accordingly, low frequency electromagnetic radiation could also be usedto stimulate varied energy absorption and acoustic responses in tissue.

FIG. 1 illustrates the acoustic wavefronts 26 produced byelectromagnetic irradiation of three absorptive regions 28 within thebreast 12. It will be understood that the acoustic waves produced byregions 28 are omnidirectional; however, for clarity only thosewavefronts directed toward coupling tank 16 have been illustrated. Theseacoustic waves travel through the tissue at a velocity of soundpropagation v_(s) which is approximately 1.5 mm/μs.

Coupling tank 16 is filled with media 29 having an acoustic impedanceand velocity of sound propagation which are close to that of a "typical"human breast. Distilled and deionized water is an effective media forthis purpose. Media 29 is retained within tank 16 by a thin sheet 30,such as polyethylene. Breast 12 is compressed against sheet 30, thusensuring good mechanical coupling from breast 12 to media 29 within tank16, and allowing acoustic energy to freely pass from breast 12 into tank16. As with sheet 19 for tank 14, good mechanical coupling through sheet30 and the similar acoustic characteristics of breast 12 and media 29enhances transmission of acoustic signals out of breast 12 and intomedia 29 and reduces acoustic wave reflections at the surface of breast12.

An array 32 of N acoustic transducers is located in tank 16. Severaluseful array geometries are discussed herein and can be usedsuccessfully in the embodiment of respect to the breast. Accordingly,the array should be at least about two inches across, and might for someapplications be as large as twelve inches across. The transducers shouldbe evenly spaced across the array. FIG. 2, for example, is a viewillustrating an essentially planar array 32, approximately three inchessquare, bearing forty-one individual transducers 33 which can be used asthe transducer array 32 in tank 16 of FIG. 1. Other arrangements oftransducers will be discussed below.

Transducers in array 32 detect acoustic pressure waves that are inducedwithin the breast by the short irradiation pulse, and travel fromemission sites (e.g., regions 28) at the velocity of sound in tissue.The transducers are fabricated so as to be most sensitive to sonicfrequencies just below the maximum frequency stimulated by theirradiation pulse noted above.

The N transducers in array 32 are coupled through N electronic signallines 34 to a computer circuit 36. Computer 36 is further connectedthrough a control line 38 to activate microwave generator 22 to producea pulse of microwave energy. Following each pulse of radiation, thetime-dependent, acoustic pressure signals recorded by each of the Ntransducer elements are electronically amplified, digitized and storedwithin computer 36. The recorded pressure signal from transducer i willbe referenced hereafter as p_(i) (t).

For sufficient resolution, the pressure signals should be digitized to aresolution of 8-12 bits at a sampling rate of at least 5-20 MHz, buthigher resolutions and sampling rates could be used. The amplifiershould have sufficient gain so that the analog thermal noise from thetransducer is greater than 1/2 LSB of the span of the analog-to-digitalconverter, or greater. Assuming the amplifier/transducer circuit has anequivalent resistance of 50 Ohms, and the amplifier has a bandwidth ofapproximately 4 MHz, thermal noise will produce a signal magnitude ofapproximately 2 μvolts. Suitable resolution can be achieved byamplifying transducer signals with a 5 MHz, 54 dB preamplifier availablefrom Panametrics, and digitizing the amplified signals with an 8-bit, 20MHz sampling rate analog-to-digital converter with a ±0.2 volt inputspan, manufactured by Gage Electronics. Additionally, adjustable highpass filtering at 0.03, 0.1 and 0.3 can be added as needed to achievedesired signal to noise performance.

As an example, FIG. 3 illustrates the pressure signals p_(i) (t) thatmight be produced by four hypothetical transducers in response topressure waves produced by a short duration of electromagneticirradiation of tissue. FIG. 3 shows the signal E(t) produced by computercircuit 36 (FIG. 1) on control line 38, which has a brief pulse, whichcauses microwave generator 22 to produce a corresponding pulse ofmicrowave energy. The resulting acoustic signals produced within breast12 are subsequently received by each of the transducers, producingsignals p_(i) (t) having differing relative magnitudes and timing, asillustrated.

It is important that the transducers be small enough so that they aresensitive to sonic waves that impinge upon the transducers from a wideangle. Referring to FIG. 4, three hypothetical absorbing regions 28a,28b and 28c are shown in greater detail, along with the respectivelycorresponding wavefronts 26a, 26b and 26c emitted by these regions,toward a transducer 33. Upon irradiation, each region 28 is the originof an acoustic pressure wave that travels in all directions. Part ofeach wave reaches transducer 33 after a delay time.

Transducer 33 is a piezoelectric ceramic chip (or a suitablealternative) having a cross-sectional diameter d exposed to regions 28a,28b and 28c. Electrical contacts (not shown) attached to the exterior oftransducer 33 detect an electrical waveform produced by the chip inresponse to mechanical vibration, as a result of the piezoelectricproperty of the ceramic chip.

Because the acoustic energy is transmitted in a wave, transducer 33 isnot equally sensitive to the pressure waves from the three absorptiveregions. The transducer is most sensitive to acoustic waves from region28c, which lies on axis 40 of transducer 33 (axis 40 being defined bythe direction that lies at a 90° angle to the front surface oftransducer 33). Transducer 33 is less sensitive to acoustic waves fromregion 28b because this region is off of axis 40. Past a certain maximumangle, θ, away from axis 40, transducer 33 is substantially insensitiveto pressure waves such as those from region 28a.

Maximum angle θ is given approximately by the relationship sin(θ)≈v_(s)τ/d, where v_(s) is the velocity of sound in the relevant medium (here,tissue), τ is the irradiation pulse length and d is the diameter of thetransducer. If a relatively large volume is to be imaged, then θ shouldbe as large as possible (small d), but if d is too small, the transducerwill produce a signal too weak to be electrically detectable withoutexcessive noise. In general, the transducer diameter should be in therange of v_(s) τ<d<4v_(s) τ. The velocity of sound in tissue isapproximately 1.5 mm/μs. Thus, for a nominal pulse width, τ, of 1 μs, dshould be in the range of approximately 1.5 to 6.0 millimeters.

FIG. 5 illustrates a second embodiment of the invention identical instructure to FIG. 1 with the exception that a pulsed source 44 ofvisible or infrared radiation 46 is used to irradiate the breast 12instead of a microwave antenna. Also, a coupling media may not be neededdue to the close m1.064 μm, pulse width<10 nsec, 250 mJ/pulse),positioned approximately 50 mm from the regions in the tissue to beimaged and collimated to a 25-100 mm diameter beam. Alternatively,radiation source 44 may be a flashtube energized by a pulsing powersupply, such as a xenon flashtube and power supply from Xenon Corp.,Woburn, Mass., which can produce a radiation pulse with a 1 μsec risetime, followed by a decaying tail with a 4 μsec time constant. Acylindrically curved, reflective surface (e.g., from Aluminum foil) maybe used with the flash tube to direct radiation from the flash tube intothe breast 12.

As noted above, array 32 is preferably of a sufficient size to image asubstantial area of tissue. In some applications, however, the tissue tobe imaged may be larger than array 32. Referring to FIG. 6, in suchsituations, array 32 and the radiation source (antenna 22 or laser orflashlamp 44) may be synchronously scanned in a rectilinear fashion asindicated by arrows 46 and 48. At each respective position of theradiation source and array 32, photoacoustic data is collected and usedto develop a corresponding image. The images may then be combined orsuperimposed to produce a complete image of the breast 12. In thisembodiment, scanning the transducer array produces the effect ofincreasing the transducer array size, and increases the angular samplingof the breast by the transducer array.

Referring to FIG. 7, in another alternative embodiment of the presentinvention, the transducer array 32 is rotated during the dataacquisition, as indicated by arrow 50. Here again, the breast 12 isirradiated by microwave, visible or infrared radiation from an antenna22, or laser or flash tube 44. At each angular position of thetransducer array, photoacoustic data is collected by the transducers andused to develop a corresponding image. The images may then be combinedor superimposed to produce a complete image of the breast 12. In thisembodiment, rotating the array 32 has the effect of increasing theeffective number of transducer elements.

FIG. 8 illustrates a specific embodiment of a rotating trans arearranged in a spiral pattern on a spherically curved surface 52. Theradius of curvature of the surface 52 is R and the diameter of the arrayis D.

The position of each of the transducers in the spiral array, relative tothe center C of curvature of surface 52, can be detailed with referenceto FIG. 8. The position of each transducer 33 is given by threespherical coordinates (r,θ,φ) as is illustrated in FIG. 8. Each of the Ntransducers 33 is on the spherical surface (at a radius R), located at aunique (θ,φ), and is oriented on the surface with its axis 40 (see FIG.4) passing through the center C of the radius of curvature of thespherically curved surface 52. The φ positions of the transducers 33range from a minimum angle of φ_(min) to a maximum angle of φ_(max). Itis desirable to maximize this range of angles, i.e., so that φ_(max)-φ_(min) is as large as possible, since doing so will enhance the extentto which features in the imaged tissue can be reconstructed in multipledimensions. (In some embodiments, φ_(max) -φ_(min) typically must beless than 45°; however, in the embodiment of FIG. 13, φ_(max) -φ_(min)approaches 90°.)

The spiral array will be rotationally stepped to each of M positionsduring data acquisition, uniformly spanning 0<θ<360°. The (θ,φ)positions of each of the N transducers are chosen so that afterscanning, the locus of N×M transducer locations produced by the Mrotational steps are distributed approximately uniformly over thespherical surface.

To accomplish uniform distribution of transducer locations over thespherical surface of the array, the θ-positions of the transducers aregiven as θ_(i) =i·(360/N)·(k+(sinθ_(min) /sinθ_(max))), where θ_(i) isthe θ-position of the i-th transducer (1<i<N), and k is an arbitraryinteger. The φ-positions of the transducers are given recursively asφ_(i+1) =φ_(i) +(α/sin(φ_(i)), where α is a constant that depends on theradius of curvature of the spherical array and the diameter of thetransducer, and φ₁ =φ_(min).

Two features of the rotationally scanned, spherical-spiral array areillustrated in FIGS. 9 and 10. FIG. 9 illustrates the convergence of theaxes 40 of the N transducers 33 to a single point within the breast. Theconvergence insures that the regions to which each of the N transducersis most sensitive (see FIG. 4) will have a high degree of overlap, in anarea 54 centered within the tissue under study. Also evident is the widerange of angles φ spanned by the transducer array.

FIG. 10 illustrates the nearly uniform distribution of the locus oftransducer locations produced by rotation of a spherically curvedsurface 52 containing an array of N=32 transducers arranged in a spiral,when stepped to 32 evenly spaced angles of rotation θ in accordance withthe foregoing. Referring to FIG. 10, one position of the 32 transducerelements is shown in cross-hatching. The remaining 31 positions of thetransducers arrived at by θ rotation of surface 52, are illustrated inoutline. As is apparent from FIG. 10, a nearly uniform distribution ofthe transducer locations across the spherical surface is achieved.

FIGS. 11A and 11B illustrate a more specific embodiment of theinvention, incorporating a spherically curved spiral transducer array.Tank 16 containing acoustic media is shaped to allow the tank to bebrought alongside the body 56 of a patient to be examined. The breast 12of the patient is compressed against the flexible sheet 30 to facilitateacoustic imaging. A source of radiation, either microwave, visible orinfrared, is placed in contact with the opposite side of the breast 12to stimulate photoacoustic waves from the breast tissue. Transducers 33are mounted on a spherically curved surface 52 such that their axes aredirected toward the center of the radius of curvature of the surface 52,resulting in a large region of sensitivity overlap as previouslyillustrated in FIG. 9.

The spherical array 52 is rotated by a stepper motor on a support shaft50 which is journalled within tank 16. A suitable stepper motorcontroller (PC board) can be obtained from New England AffiliatedTechnologies. The transducer array may be formulated from a monolithic,annular array of five mm diameter elements, arranged in a spiral patternas discussed above. Satisfactory results have been achieved using low-Qceramic transducers having a wide band frequency response from 200 kHzto 2 MHz, falling to zero near 4 MHz.

The annular array is encased in an aluminum-shielded housing in whichpreamplifiers and line drivers are incorporated. Referring to FIG. 12, asuitable amplifier circuit can be constructed from a JFET 57 and bipolartransistor 59 arranged in a dual-stage amplifier. Signals output fromthe integral amplifier/line drivers are led outside of tank 16 usingultra-thin coaxial cable cables, to an external amplifier andanalog-to-digital converter.

FIG. 13 illustrates another embodiment of the invention, specificallyadapted for human breast imaging, in which the angle φ_(max) -φ_(min) ofspherically curved surface 52 is substantially larger than in thepreceding embodiment. In this embodiment, the microwave source is ahelical, "end-launch" antenna 20, for which the spherically curved,conductive surface of the spherical transducer array 52 serves as aground plane. Surface 52 also serves as a tank for containing anacoustic and electromagnetic coupling media 18/29. (Distilled anddeionized water serves as a suitable acoustic/electromagnetic couplingmedia.) The breast is suspended vertically into the coupling media 18/29as illustrated, to permit coupling of both microwave energy into thebreast and acoustic energy out of the breast. The individual transducers33 are arranged as a spherical, spiral array as previously described,and the surface 52 is rotated on shaft 50 to collect an evendistribution of samples from the transducers.

After sonic pressure waves are recorded using one of the embodiments ofthe invention described above, photoacoustic images must be"reconstructed" from multiple pressure signals. The aim is toreconstruct some property of the breast from an ensemble of pressuremeasurements made externally to the breast. In this case, thesemeasurements are time-dependent pressure signals recorded subsequent toobject-irradiation by a short pulse of radiation.

The generalized reconstruction geometry is illustrated in FIG. 14. Theexcess pressure p(r,t) that arrives at position r, where transducer 33is located, at time t, is the sum of the pressure waves produced at allpositions within the tissue. This sum can be expressed as a volumeintegral: ##EQU1## where ρ is the mass density and β is the coefficientof thermal expansion of the tissue, the volume integral is carried outover the entire r'-space where the temperature acceleration ∂²T(r',t')/∂t'² is non-zero, and where t'=t-|r-r'|/v_(s) (|r-r'|/v_(s)being the time delay for an acoustic pressure wave to propagate fromposition r' to position r at the speed of sound in tissue v_(s)).

Under the assumption that the radiation pulse which causes thetemperature acceleration is of a duration τ which is short enough (τ<1μs) to generate an adiabatic expansion of absorptive tissue, thepreceding equation can be rewritten in terms of a regional heatabsorption function S(r',t): ##EQU2## where C is the specific heat oftissue. We can further write the heating-function as the product of apurely spatial and a purely temporal component, i.e.,

    S(r',t')=I.sub.0 R(r')T(t')                                (3)

where I₀ is a scaling factor proportional to the incident radiationintensity and R(r') represents the fractional energy absorption of r'.Defined in this way I₀ T(t') describes the irradiating field and R(r')describes the absorption properties of the medium (breast). The excesspressure can then be written as: ##EQU3##

Equation 4 expresses how the time-sequential information conveyed by thepressure signal delivers spatial information about the absorptionproperties of the medium.

To further simplify, both sides of equation (4) are integrated in timeand multiplying factors are moved to the left, to obtain: ##EQU4##

Now, assuming that the temporal distribution of the irradiating field isof unit height and duration τ (see the function E(t) illustrated in FIG.3), T(t') has a value of 1 only from t'=0 to t'=τ. As a result, theintegrand on the right side of equation (5) will have a value of zeroeverywhere except along a thin, spherical "shell" of inner radius v_(s)t surrounding point r, where 0<t'<τ, i.e., where |r-r'|/v_(s)<t<τ+|r-r'|/v_(s). This thin "shell" has a thickness of v_(s) τ;accordingly, the volume integral for this thin "shell" can beapproximated by v_(s) τ multiplied by the surface integral, over theinner surface of the "shell", i.e., where |r-r'|/v_(s) =t, i.e.:##EQU5## Finally, noting that |r-r'|=v_(s) t, and rearranging terms, wecan define the "projection" at the position r, S_(r) (t), as ##EQU6##

Equation (7) shows that the integral of all pressure waves received at atransducer at position r and up to time t, is proportional to the sum ofthe absorption function over a spherical surface a distance v_(s) t fromthe transducer. Accordingly, an image of R(r') can be reconstructed bymapping integrated pressure data acquired at multiple transducers, overspherical surfaces (to create three-dimensional image) or co-planar arcs(to create a two-dimensional image).

Specifically, referring to FIG. 15, this method of image reconstructioncomprises:

1. Positioning transducers acoustically coupled to the tissue understudy (step 60).

2. Positioning an electromagnetic source electromagnetically coupled tothe tissue under study (step 62).

3. Irradiating the tissue with a brief pulse of electromagnetic energyE(t) at time t=0 to induce acoustic signals in the tissue (step 64).

4. Sampling and storing pressure measurements P_(i) (t) at eachtransducer i beginning at time t=0 (step 66).

5. Computing the sums ##EQU7## of pressure signals (step 68). 6. For apoint r' in the tissue to be imaged, determining the time delay t_(i)needed for sound to travel from point r' to the position r_(i) oftransducer i (step 70), selecting the value of the sum S_(i) (t_(i))(generated from transducer i) which occurs at time t_(i) (step 72),repeating these steps for each transducer i (step 74), and thenaccumulating the selected values S_(i) (t_(i)) to generate a value K(r')at position r' according to ##EQU8## (step 76). 8. Repeating step 7 foreach point r' to be imaged (step 78).

9. Spatially filtering the resulting values of K(r') to obtain valuesfor R(r'). This filtering can be performed in the frequency domain usinga function having a response proportional to the square of frequency.Alternatively, filtering may be performed by computing the Laplacian ofthe three-dimensional spatial function K(r'), i.e., R(r')=A.∇² K(r') (9)(Step 70).

9. Plotting the values of R(r') as an image of the tissue (step 82).

This reconstruction methodology was generally tested for atwo-dimensional image, by constructing the simplified experimental testbed illustrated in FIG. 16. The test bed included a wideband transducer82 with a center frequency of 2 MHz, mounted on a 150 mm arm that wasrotated along a circular path 84 under stepper-motor control. Thetransducer was 50 mm (height)×6 mm (width) and had a radius of curvatureof 150 mm along the long dimension. The transducer was asymmetrical andfocused in one dimension radially inwardly with respect to path 84;accordingly, the transducer was most responsive to acoustic signalsreceived over a wide angle within the horizontal plane of circular path84.

The scanning mechanism was immersed in a 50 ml/l concentrationIntralipid-10%, a fatty emulsion frequently used as a tissue-mimickingscattering medium. The scattering coefficient (μ_(s)) for Intralipid-10%@ 1.064 μm was measured as 0.015 mm⁻¹ /ml/l. This is close to the 0.013mm⁻¹ /ml/l reported by van Staveren. (See van Staveren, H. J., et al.,"Light scattering in Intralipid-10% in the wavelength range of 400-1100nm", Applied Optics, 31(1):4507-4514 (1991).) Using a value of 0.48 forthe mean cosine of scatter (g), as reported by van Staveren, and thescattering coefficient measured in our laboratory, the 50 ml/lconcentration of Intralipid-10% produced a reduced scattered coefficientμ_(s) '=0.39 mm⁻¹ μ_(s) '≡(1-g)μ_(s) !. At this wavelength, theabsorption of Intralipid-10% is due almost entirely to the absorption ofwater, μ_(a) ≡0.0164 mm⁻¹⁹. These values are a factor of 2-3 less thanthose measured in vitro for different types of breast tissue at 900 nm.

A 50 mm diam laser beam from a pulsed Nd:YAG laser (λ=1.064 μm, pulsewidth <10 ns, 20 Hz repetition rate, 250 mJ/pulse) illuminated thescattering medium from below. The imaging plane of path 84 was normal tothe laser beam and was located 47.5 mm above the bottom surface of thescattering medium. The laser beam axis and rotational axis of thetransducer scanning arm were coincident.

Data acquisition proceeded as follows: The transducer was steppedthrough 360° at 2° increments along path 84. At each angle, the temporalacoustic signal recorded by the transducer was digitized to 12 bits at arate of 10 MHz for a total of 1024 samples. The sampling interval wassynchronized to the pulsing of the laser. At each angle, the temporalacoustic signal for 16 consecutive pulses were averaged. This procedurewas repeated for 180 angles.

The absorption phantom illustrated in FIG. 16 was used in imaging. Itconsisted of a 4 mm diam, black, latex ball 86 and a black, rubbercylinder 88 suspended on two, 0.35 mm diam, clear, polyethylene threads.The dimensions for the cylinder were 8.5 mm outside diameter 5.0 mminside diameter and 4 mm length.

Image reconstruction proceeded using an adaptation of the integrated,filtered-back projection algorithm described above, applicable to atwo-dimensional image. The S_(r) (t) were computed for each of the 180transducer angles, backprojected over appropriate arcs and summed. Avalue of v_(s) =1.5 mm/μs was assumed. The next step was to apply a 2-Dfilter. Filtering was performed in the frequency domain using a linearramp function a cosine-weighted apodizing window, i.e., F(f)=|f/f_(n)|*(1+cos(πf/f_(n)))/2, where f is the spatial frequency and f_(n) is theNyquist frequency associated with the reconstruction matrix. In thisinstance, f_(n) =3 cycles/ mm. The center 30 mm region of thereconstruction is displayed in FIG. 17.

The basic relationship between an acoustic signal and a heterogeneousdistribution of absorbed energy is given by Equation 7. At any moment intime following an irradiating optical pulse, the temporally weighted andtemporally integrated acoustic pressure up to that time is proportionalto a surface integral of the absorbed heat distribution R(r) within theobject being imaged. This relationship is true, provided the irradiatingoptical pulse is short enough and sharp enough. This condition is metfor optical pulses less than 1 μs duration.

In order to "reconstruct" R(r') from a set of acoustic measurements,data must be sampled over at least 2π steradians. In the restrictedcase, where significant optical absorption takes place within a narrowplane, R(r') can be reconstructed using a set of co-planar acoustic dataacquired over 360°. The image displayed in FIG. 17 was reconstructedunder these conditions. This image reflects what one would expect: a"cut" through the center of a spherical and cylindrical absorber. It isof note that a "halo" artifact surrounds the image of the latex ball 86.This originates from the decreased velocity of sound within the latexball (1.0 mm/μs) compared to the Liposyn-10% solution (1.5 mm/μs).

Were R(r') distributed throughout a larger volume, it would have beennecessary to obtain acoustic data over the surface of a hemisphere inorder to adequately reconstruct R(r'). Such an operation can beperformed by the transducer geometries described above.

Further details on the above experimental arrangement can be found inKruger et al., "Photoacoustic ultrasound (PAUS)--Reconstructiontomography", Medical Physics 22(10):1605-1609 (October 1995),incorporated by reference herein in its entirety.

A second methodology for image generation can also be derived fromEquations (8) and (9). Specifically, it can be shown that the Laplacianof the back-projection of the time-weighted, integrated pressure signalsis approximately equal to the backprojection of the first timederivative of the pressure signal, if the radius R of any imaged objectis small, i.e., where |r-r'|>>R, as follows: ##EQU9## where t_(i)=|r_(i) -r'|/v_(s), r' is a vector that denotes the location within thetissue, r_(i) is a vector that denotes the location of transducer i,v_(s) is the velocity of sound, A is a constant, and p_(i) (t_(i)) isthe samples of the pressure signal that reaches the i-th transducer.

Referring to FIG. 18, using this approximation, the steps in thereconstruction process are as follows:

1. Positioning transducers acoustically coupled to the tissue understudy (step 114).

2. Positioning an electromagnetic source electromagnetically coupled tothe tissue under study (step 116).

3. Irradiating the tissue with brief pulse of electromagnetic energyE(t) at time t=0 to induce acoustic signals in tissue (step 118).

4. Sampling and storing pressure measurements p_(i) (t) at eachtransducer beginning at time t=0 (step 120).

5. Calculating the time-weighted, first temporal derivative of P_(i)(t_(i)), i.e., t_(i) (dp_(i) (t_(i))/dt), for each of the i transducers(step 122).

6. For each position, r', in the tissue, summing the selected values ofthe time-weighted first temporal derivatives of the pressure signalsfrom each transducer as indicated in Equation 9 (steps 124-132).

7. Generating an image of the tissue from computed values of R(r') (step134).

This reconstruction procedure produces three-dimensional images of theenergy deposition within the interior of the tissue, which isrepresentative of the differential absorption of the irradiating energyby the different types of tissues within the tissue.

To perform the above calculation, it is necessary to obtain the firsttime-derivative of the pressure signal that reaches each transducer. Itshould be noted, however, that a transducer produces a characteristic"ringing" in its electrical response to an externally-applied pulse ofpressure, which distorts the shape of the electrical output of thetransducer away from that of the pressure waveform. Referring to FIG.19, this ringing response 136 approximates the impulse response of thetransducer 33, i.e., the electrical signal as a function of time that isproduced when a very abrupt pressure impulse 138 strikes the transducer.

If a transducer were fabricated to produce a simple biphasic (or"doublet") response to an a impulse of pressure, that is one positivelobe, followed a short time later by one negative lobe (an idealresponse 136 is illustrated in FIG. 19), then the electrical output ofthe transducer would be approximately proportional to the firsttime-derivative of the input pressure signal. This would be desirable,because it would eliminate the necessity of computing the firsttime-derivative of the input pressure signal; rather, the timederivative would be produced by the transducer in the first instance.

For any real transducer, however, such a response would be difficult toachieve. Rather, the impulse response of a transducer is closer to adamped sinusoid, as is illustrated in waveform 140 (p(t)) in FIG. 20A.In this example, the impulse response of the transducer is assumed to beof the form p(t)=sin(2πft)e.sup.αft. Such a response displays a periodiccomponent of a characteristic temporal frequency f, that decaysexponentially with time.

In this case, an approximate "differential" transducer response can bysynthesized by delaying the originally recorded pressure waveform, p(t),by varying amounts, weighing the delayed pressure signals, and summingthe delayed pressure signals together with the original waveform. Anexample is illustrated in FIG. 20A-20B, which shows two weighted,time-delayed waveforms (Ap(t-Δt) 142 and Bp(t-2Δt) 144 (where Δt is1/2f) generated from the assumed impulse response 140 of the transducer.When the time-delayed waveforms 142 and 144 are added to the response140 of the transducer, the resulting waveform 146 (FIG. 20D) synthesizesa biphasic impulse response S(t).

Thus, to implement the reconstruction algorithm described above, thetransducer responses can be synthesized to be differential in natureusing the methodology illustrated in FIG. 20A-20D, after which theoutput of each transducer will be proportional to dp(t)/dt.

Referring to FIG. 21, a circuit for performing such a reconstructionincludes an analog-to-digital converter 148 for converting the analogsignal from the transducer to an equivalent digital signal, an amplifier149 and cache 150 for receiving and temporarily storing samples from A/Dconverter 148 and outputting the sample which was stored Δt earliermultiplied by a gain factor A, a second amplifier 151 and cache 152 forstoring samples and outputting the sample which was stored 2Δt earliermultiplied by a gain factor B, and a digital accumulator 154 for summingthe outputs of caches 148 and 150 with the current sample from the A/Dconverter to produce an output digital signal S which is representativeof dp(t)/dt.

Using a circuit such as that shown in FIG. 21, steps 120 and 122 of thereconstruction process described by FIG. 18 can be accomplished in asingle operation by hardware rather than in software computations,increasing the scanning and imaging rate of the apparatus.

While the present invention has been illustrated by a description ofvarious embodiments and while these embodiments have been described inconsiderable detail, it is not the intention of the applicants torestrict or in any way limit the scope of the appended claims to suchdetail. Additional advantages and modifications will readily appear tothose skilled in the art. The invention in its broader aspects istherefore not limited to the specific details, representative apparatusand method, and illustrative example shown and described. Accordingly,departures may be made from such details without departing from thespirit or scope of applicant's general inventive concept.

What is claimed is:
 1. A method of imaging tissue structures in athree-dimensional volume of tissue by detecting localized absorption ofelectromagnetic waves in said tissue, comprisingproviding a source ofelectromagnetic radiation in proximity to said tissue; providing aplurality of acoustic sensors; acoustically coupling said plurality ofacoustic sensors to said tissue; irradiating said three-dimensionalvolume of tissue with a pulse of diffuse electromagnetic radiation fromsaid source to generate resultant pressure waveforms within saidthree-dimensional volume of tissue; detecting said resultant pressurewaveforms arriving at said acoustic sensors and storing datarepresentative of said waveforms; combining a plurality of said detectedpressure waveforms to derive a measure of pressure waveforms originatingat a point distant from said acoustic sensors; and repeating saidcombining step for a plurality of points to produce an image ofstructures in said tissue.
 2. The method of claim 1 wherein said step ofcombining a plurality of detected pressure waveforms to derive a measureof pressure waveforms originating at a point comprisesdetermining adistance between said point and a pressure sensor, computing a valuerelated to the time rate of change in a pressure waveform detected bysaid pressure sensor, at a time which is a time delay after said pulseof electromagnetic radiation, said time delay being equal to the timeneeded for sound to travel said distance through said tissue; repeatingsaid determining and computing for additional pressure sensors andpressure sensor waveforms; and accumulating said computed values to formsaid measure of pressure waveforms originating at said point.
 3. Themethod of claim 2 whereinsaid step of providing a plurality of acousticsensors comprises providing a differentiating acoustic sensor responsiveto a pressure waveform by producing an electrical output representativeof a time rate of change of said pressure waveform, and said step ofcomputing a value of the time rate of change in a pressure waveform,comprises computing a value of said electrical output of saiddifferentiating pressure sensor.
 4. The method of claim 3 whereinsaiddifferentiating acoustic sensor includes a piezoelectric crystal whichproduces an analog signal, and producing an electrical outputrepresentative of a time rate of change comprises combining a delayedversion of said analog signal with said analog signal to produce saidelectrical output.
 5. The method of claim 2 wherein computing a valuerelated to the time rate of change in a pressure waveform at a timedelay, further comprises multiplying said time rate of change by afactor proportional to said time delay to produce said value,whereby tocompensate for diffusion of acoustic energy radiated from said point. 6.The method of claim 1 wherein said step of combining a plurality ofdetected pressure waveforms to derive a measure of pressure waveformsoriginating at a point, comprisesdetermining a distance between saidpoint and a pressure sensor; computing a value related to a sum of thepressure waveform detected by said pressure sensor over a time period,said time period beginning substantially contemporaneous with said pulseof electromagnetic radiation and said time period having a durationequal to the time needed for sound to travel said distance through saidtissue; repeating said determining and computing for additional pressuresensors and pressure sensor waveforms; and accumulating said sums toform said measure of pressure waveforms originating at said point. 7.The method of claim 6 wherein computing a value related to a sum of thepressure waveform detected by a pressure sensor over a time period,further comprises multiplying said sum by a factor proportional to theduration of said time period to produce said value,whereby to compensatefor diffusion of acoustic energy radiated from said point.
 8. The methodof claim 1 wherein providing said plurality of sensors comprisesproviding a surface and positioning said sensors evenly spaced acrosssaid surface.
 9. The method of claim 8 wherein said steps of irradiatingsaid tissue and detecting said pressure waveforms are performed whilesaid surface and said sensors are at a first position, and furthercomprising the steps ofmoving said surface and said sensors to a secondposition, repeating said irradiating step, repeating said detectingstep, and combining waveforms collected by said sensors in said firstand said second positions to generate said image of said tissue.
 10. Themethod of claim 9 wherein moving said surface comprises moving saidsurface in a rectilinear fashion.
 11. The method of claim 9 furthercomprising moving said electromagnetic radiation source in synchronywith said surface and said sensors.
 12. The method of claim 9 whereinmoving said surface comprises rotating said surface.
 13. The method ofclaim 12 wherein said sensors are positioned on said surface along aspiral path.
 14. The method of claim 1 further comprising immersing saidsensors in an acoustic coupling media, said acoustic coupling mediahaving an acoustic characteristic impedance which is substantiallysimilar to that of said tissue to reduce reflections of acoustic wavesimpinging into said media from said tissue.
 15. The method of claim 14further comprisingproviding a flexible film containing said acousticcoupling media, and pressing said tissue upon said flexible film tocouple acoustic waves from said tissue into said acoustic couplingmedia.
 16. The method of claim 14 further comprising immersing saidelectromagnetic radiation source in an electromagnetic coupling media,said electromagnetic coupling media having an electromagneticcharacteristic impedance which is substantially similar to that of saidtissue to reduce reflections of electromagnetic waves impinging intosaid tissue from said electromagnetic coupling media.
 17. The method ofclaim 16 further comprisingproviding a flexible film enclosingelectromagnetic coupling media, and pressing said tissue upon saidflexible film to couple electromagnetic waves from said electromagneticcoupling media into said tissue.
 18. The method of claim 14 furthercomprisingimmersing said electromagnetic radiation source in saidacoustic coupling media, wherein said acoustic coupling media has acharacteristic electromagnetic impedance which is substantially similarto that of said tissue, to reduce reflections of electromagnetic wavesimpinging into said tissue from said media.
 19. The method of claim 1wherein irradiating said tissue comprises irradiating said tissue with alaser generating electromagnetic radiation in the near-infrared band.20. The method of claim 1 wherein irradiating said tissue comprisesirradiating said tissue with a Xenon flash lamp.
 21. The method of claim1 wherein irradiating said tissue comprises irradiating said tissue withan electrically conductive coil generating microwave frequencyradiation.
 22. The method of claim 21 wherein said microwave frequencyis substantially four hundred and thirty-three MHz.
 23. The method ofclaim 21 wherein said microwave frequency is substantially nine hundredand fifteen MHz.
 24. Apparatus for imaging tissue structures in athree-dimensional volume of tissue by detecting localized absorption ofelectromagnetic waves in said tissue, comprisingan electromagneticradiation source; a plurality of acoustic sensors arrayed across asurface, said surface being acoustically coupled to said tissue; powercircuitry pulsing said electromagnetic radiation source to produce apulse of diffuse electromagnetic radiation from said source irradiatingsaid three-dimensional volume of tissue to generate resultant pressurewaveforms within said three-dimensional volume of tissue; and computingcircuitry detecting resultant pressure waveforms arriving at saidacoustic sensors, storing data representative of said waveforms, andcombining a plurality of said detected pressure waveforms to derive animage, points in said image being derived by combining measures ofpressure waveforms originating at points within said tissue.
 25. Theapparatus of claim 24 wherein said sensors are piezoelectric transducershaving a largest dimension smaller than four times the distance traveledby sound in tissue over the time duration of said pulse ofelectromagnetic radiation.
 26. The apparatus of claim 24 wherein saidsensors are evenly spaced across said surface.
 27. The apparatus ofclaim 24 further comprising a motor coupled to said surface for movingsaid surface and said sensors to generate said image of said tissue. 28.The apparatus of claim 27 wherein said motor moves said surface in arectilinear fashion.
 29. The apparatus of claim 28 further comprising asecond motor coupled to said electromagnetic radiation source for movingsaid source in synchrony with said surface and said sensors.
 30. Theapparatus of claim 27 wherein said motor rotates said surface.
 31. Theapparatus of claim 30 wherein said sensors are positioned on saidsurface along a spiral path.
 32. The apparatus of claim 24 furthercomprisinga tank containing acoustic coupling media, said surface beingpositioned inside of said tank and immersed in said acoustic couplingmedia, whereby said tank may be filled with an acoustic coupling mediahaving an acoustic characteristic impedance which is substantiallysimilar to that of said tissue to reduce reflections of acoustic wavesimpinging into said media from said tissue.
 33. The apparatus of claim32 wherein said tank includes an open top surface whereby said tissuemay be received into said acoustic coupling media.
 34. The apparatus ofclaim 32 wherein said tank further comprises a flexible film coverenclosing said tank to contain said acoustic coupling media, wherebysaid tissue may be pressed upon said flexible film to couple acousticwaves into said acoustic coupling media.
 35. The apparatus of claim 32further comprising a second tank containing an electromagnetic couplingmedia,said electromagnetic radiation source being positioned inside ofsecond tank and immersed in said electromagnetic coupling media, wherebysaid second tank may be filled with an electromagnetic coupling mediahaving an electromagnetic characteristic impedance which issubstantially similar to that of said tissue to reduce reflections ofelectromagnetic waves impinging into said tissue from saidelectromagnetic coupling media.
 36. The apparatus of claim 35 whereinsaid second tank further comprises a flexible film cover enclosing saidtank to contain said electromagnetic coupling media, whereby said tissuemay be pressed upon said flexible film to couple electromagnetic wavesfrom said electromagnetic coupling media into said tissue.
 37. Theapparatus of claim 32 wherein said electromagnetic radiation source ispositioned inside of said tank and immersed in said acoustic couplingmedia,whereby said acoustic coupling media in said tank may be selectedto have a characteristic electromagnetic impedance which issubstantially similar to that of said tissue, to reduce reflections ofelectromagnetic waves impinging into said tissue from said media. 38.The apparatus of claim 24 wherein said electromagnetic radiation sourceis a laser.
 39. The apparatus of claim 38 wherein said laser emitselectromagnetic radiation in the near-infrared band.
 40. The apparatusof claim 38 wherein said laser is a Nd:YAG laser.
 41. The apparatus ofclaim 24 wherein said electromagnetic radiation source is a flash lamp.42. The apparatus of claim 41 wherein said flash lamp is a Xenon flashlamp.
 43. The apparatus of claim 24 wherein said electromagneticradiation source is an electrically conductive coil.
 44. The apparatusof claim 43 wherein said power circuitry pulses said coil at a microwavefrequency.
 45. The apparatus of claim 44 wherein said microwavefrequency is substantially four hundred and thirty-three MHz.
 46. Theapparatus of claim 44 wherein said microwave frequency is substantiallynine hundred and fifteen MHz.
 47. A method of imaging tissue structuresby detecting localized absorption of electromagnetic waves in saidtissue, comprisingproviding a source of electromagnetic radiation inproximity to said tissue; providing a plurality of acoustic sensors;acoustically coupling said plurality of acoustic sensors to said tissue;irradiating said tissue with a pulse of electromagnetic radiation fromsaid source to generate resultant pressure waveforms within said tissue;detecting said resultant pressure waveforms arriving at said acousticsensors and storing data representative of said waveforms; combining aplurality of said detected pressure waveforms to derive a measure ofpressure waveforms originating at a point distant from said acousticsensors, by determining a distance between said point and a pressuresensor, computing a value related to a sum of the pressure waveformdetected by said pressure sensor over a time period, said time periodbeginning substantially contemporaneous with said pulse ofelectromagnetic radiation and said time period having a duration equalto the time needed for sound to travel said distance through saidtissue, repeating said determining and computing for additional pressuresensors and pressure sensor waveforms, and accumulating said sums toform said measure of pressure waveforms originating at said point; andrepeating said combining step for a plurality of points to produce animage of structures in said tissue.
 48. The method of claim 47 whereincomputing a value related to a sum of the pressure waveform detected bya pressure sensor over a time period, further comprises multiplying saidsum by a factor proportional to the duration of said time period used toproduce said value,whereby to compensate for diffusion of acousticenergy radiated from said point.
 49. A method of imaging tissuestructures by detecting localized absorption of electromagnetic waves insaid tissue, comprisingproviding a source of electromagnetic radiationin proximity to said tissue; providing a surface and positioning aplurality of acoustic sensors spaced across said surface in a spiralpath; acoustically coupling said plurality of acoustic sensors to saidtissue; positioning said surface and said sensors in a first position;irradiating said tissue with a pulse of electromagnetic radiation fromsaid source to generate resultant pressure waveforms within said tissue;detecting said resultant pressure waveforms arriving at said acousticsensors and storing data representative of said waveforms; rotating saidsurface and said sensors to a second position; repeating saidirradiating step; repeating said detecting step; combining a pluralityof said detected pressure waveforms collected by said sensors in saidfirst and said second positions to derive a measure of pressurewaveforms originating at a point distant from said acoustic sensors; andrepeating said combining step for a plurality of points to produce animage of structures in said tissue.
 50. A method of imaging tissuestructures by detecting localized absorption of electromagnetic waves insaid tissue, comprisingproviding an acoustic coupling media adjacentsaid tissue, having an acoustic characteristic impedance which issubstantially similar to that of said tissue to reduce reflections ofacoustic waves impinging into said media from said tissue; providing anelectromagnetic coupling media adjacent said tissue, saidelectromagnetic coupling media having an electromagnetic characteristicimpedance which is substantially similar to that of said tissue toreduce reflections of electromagnetic waves impinging into said tissuefrom said electromagnetic coupling media; providing a source ofelectromagnetic radiation in proximity to said tissue and immersing saidsource in said electromagnetic coupling media to electromagneticallycouple said source to said tissue; providing a plurality of acousticsensors and immersing said sensors in said acoustic coupling media toacoustically couple said plurality of acoustic sensors to said tissue;irradiating said tissue with a pulse of electromagnetic radiation fromsaid source to generate resultant pressure waveforms within said tissue;detecting said resultant pressure waveforms arriving at said acousticsensors and storing data representative of said waveforms; combining aplurality of said detected pressure waveforms to derive a measure ofpressure waveforms originating at a point distant from said acousticsensors; and repeating said combining step for a plurality of points toproduce an image of structures in said tissue.
 51. The method of claim50 further comprisingproviding a flexible film enclosing electromagneticcoupling media, and pressing said tissue upon said flexible film tocouple electromagnetic waves from said electromagnetic coupling mediainto said tissue.
 52. A method of imaging tissue structures by detectinglocalized absorption of electromagnetic waves in said tissue,comprisingproviding an coupling media adjacent said tissue, saidcoupling media having an acoustic characteristic impedance which issubstantially similar to that of said tissue to reduce reflections ofacoustic waves impinging into said media from said tissue, and having acharacteristic electromagnetic impedance which is substantially similarto that of said tissue, to reduce reflections of electromagnetic wavesimpinging into said tissue from said media; providing a source ofelectromagnetic radiation in proximity to said tissue and immersing saidsource in said coupling media to electromagnetically couple said sourceto said tissue; providing a plurality of acoustic sensors and immersingsaid sensors in said coupling media to acoustically couple saidplurality of acoustic sensors to said tissue; irradiating said tissuewith a pulse of electromagnetic radiation from said source to generateresultant pressure waveforms within said tissue; detecting saidresultant pressure waveforms arriving at said acoustic sensors andstoring data representative of said waveforms; combining a plurality ofsaid detected pressure waveforms to derive a measure of pressurewaveforms originating at a point distant from said acoustic sensors; andrepeating said combining step for a plurality of points to produce animage of structures in said tissue.
 53. Apparatus for imaging tissuestructures by detecting localized absorption of electromagnetic waves insaid tissue, comprisingan electromagnetic radiation source; a pluralityof acoustic sensors arrayed across a surface along a spiral path, saidsurface being acoustically coupled to said tissue; a motor coupled tosaid surface for rotating said surface and said sensors; power circuitrypulsing said electromagnetic radiation source to produce a pulse ofelectromagnetic radiation from said source within said tissue; andcomputing circuitry detecting resultant pressure waveforms arriving atsaid acoustic sensors, storing data representative of said waveforms,and combining a plurality of said detected pressure waveforms to derivean image, points in said image being derived by combining measures ofpressure waveforms originating at points within said tissue. 54.Apparatus for imaging tissue structures by detecting localizedabsorption of electromagnetic waves in said tissue, comprisinga firsttank containing an acoustic coupling media having an acousticcharacteristic impedance which is substantially similar to that of saidtissue to reduce reflections of acoustic waves impinging into said mediafrom said tissue; a second tank containing an electromagnetic couplingmedia having an electromagnetic characteristic impedance which issubstantially similar to that of said tissue to reduce reflections ofelectromagnetic waves impinging into said tissue from saidelectromagnetic coupling media; a plurality of acoustic sensorspositioned within said first tank and immersed in said acoustic couplingmedia; an electromagnetic radiation source positioned inside of saidsecond tank and immersed in said electromagnetic coupling media; powercircuitry pulsing said electromagnetic radiation source to produce apulse of electromagnetic radiation from said source within said tissue;and computing circuitry detecting resultant pressure waveforms arrivingat said acoustic sensors, storing data representative of said waveforms,and combining a plurality of said detected pressure waveforms to derivean image, points in said image being derived by combining measures ofpressure waveforms originating at points within said tissue.
 55. Theapparatus of claim 54 wherein said second tank further comprises aflexible film cover enclosing said tank to contain said electromagneticcoupling media, whereby said tissue may be pressed upon said flexiblefilm to couple electromagnetic waves from said electromagnetic couplingmedia into said tissue.
 56. Apparatus for imaging tissue structures bydetecting localized absorption of electromagnetic waves in said tissue,comprisinga tank containing a coupling media having an acousticcharacteristic impedance which is substantially similar to that of saidtissue to reduce reflections of acoustic waves impinging into said mediafrom said tissue, and having an electromagnetic characteristic impedancewhich is substantially similar to that of said tissue to reducereflections of electromagnetic waves impinging into said tissue fromsaid coupling media; an electromagnetic radiation source positionedinside of said tank and immersed in said coupling media; a plurality ofacoustic sensors positioned within said tank and immersed in saidcoupling media; power circuitry pulsing said electromagnetic radiationsource to produce a pulse of electromagnetic radiation from said sourcewithin said tissue; and computing circuitry detecting resultant pressurewaveforms arriving at said acoustic sensors, storing data representativeof said waveforms, and combining a plurality of said detected pressurewaveforms to derive an image, points in said image being derived bycombining measures of pressure waveforms originating at points withinsaid tissue.